Lines of Effort

Effort Lyndon

5.7 Lyndon in Hera thumbhole.

As of this writing (July 2018), Effort Lyndon is the easiest known method of building a semi-automatic closed-bolt centerfire rifle. In 5.7x28mm FN caliber, a Lyndon is comprised of an AR57 blowback upper that accepts up to 50 round magazines, an AR thumbhole stock of any make, and an AR trigger jig. With the correct trigger jig, a simple hacksaw cut is all that is required for the trigger jig to attach to the thumbhole stock. Self-ejecting single shots are also easy to make in any caliber, if somewhat difficult to load.

.22 LR single shot Lyndon in Hera thumbhole.

A .22 LR self-ejecting upper installed on the Lyndon - 1st test

The Lyndon should work as a self-ejecting single shot with any standard length AR-15 upper. .50 BMG uppers should work, albeit one should make sure the stock can handle the recoil or suitably reinforce it. Lyndons should run semi-automatically with any upper having a cartridge feeding device installed on the upper, including belt feeds.

This would seem to be the best available trigger jig for the Lyndon - no drilling is required for the rear takedown pin, just a simple hacksaw cut to allow the jig to fit in the thumbhole stock properly:


Please note the rear cut does not have to be precise. If after your first cut, the jig doesn't fit in the stock, cut again.

Known links:




SierraWhiskeyMike's semi-auto .22 LR pistol build, should also be found on Reddit:


His firing video on YouTube: